1. Field of the Invention
The present invention relates to a switching method and apparatus, a wireless network, and an access point, more particularly to an efficient method of transmitting data between access points in a wireless network.
2. Description of the Related Art
One well-known type of wireless network is a local area network (LAN) accessed through access points that communicate via wireless links with client devices and via wired links with other access points and external networks. A network of this type is constrained to operate in areas where the necessary wired links can be installed. To expand the network's service area, it would be desirable if this constraint could be removed. There is accordingly considerable interest in wireless mesh networks in which access points communicate with each other over wireless links instead of or in addition to wired links.
The communication protocols used on the wired and wireless links of conventional wireless LANs have been standardized within the framework of the Standards for Information Technology of the Institute of Electrical and Electronics Engineers (IEEE). Specifically, communications between access points conform to the IEEE 802.3 family of standards, whereas communications between access points and client devices conform to the IEEE 802.11 family of standards. The two standards use different data formats: IEEE 802.3 communication takes place in basic units referred to as packets; IEEE 802.3 communication takes place in different basic units referred to as data frames. A known way to accommodate the difference is for an access point that receives a data frame from a client device to encapsulate the data frame in a packet for transmission to another access point. When the packet reaches its destination access point, the data frame is de-encapsulated and sent to the destination client device.
As the wireless links in a network multiply, collisions between transmitted data become increasingly likely, and access to the network must be controlled accordingly. In one widely used control method, known as carrier sense multiple access/collision avoidance (CSMA/CA), before transmitting on a wireless channel, a device tests the status of the channel. If the channel is idle, the device proceeds to transmit; if the channel is busy, the device waits awhile, then tests the channel again. CSMA/CA protocols are included in the IEEE 802.11 standards. A Japanese description is given in Handa, ‘2 IEEE 802.11 hoshiki musen LAN no MAC reiya gijutsu’ (MAC layer technology for IEEE 802.11 wireless LANs), CQ Publishing Co., Ltd., February 2003, available online as of Feb. 3, 2005 at http://www.cqpub.co.jp/INTERFACE/toku/2003/200302/toku3.htm.
The IEEE 802.11 standards call for data to be transmitted in frames with a specified interframe space (IFS), which prevents any one device from monopolizing a wireless channel. The standards also specify a back-off algorithm to determine how long a device must wait, after finding that the channel is busy, before attempting to transmit again. The back-off time is randomized to prevent collisions that would occur if multiple access points all began to transmit on an idle channel simultaneously. Similar spacing and back-off requirements are part of the IEEE 802.3 wireless standards.
When each IEEE 802.11 data frame is encapsulated in a separate IEEE 802.3 packet, the need to add a separate IEEE 802.3 header to each packet, and to comply with interframe space and back-off requirements, creates an inefficient flow of data between access points. This is particularly true when the traffic between access points and client devices includes many short data frames, as is the case when telephone calls are transmitted by the Voice over Internet Protocol (VoIP), for example.